Foot-and-mouth disease (FMD) is one of the most virulent and contagious diseases affecting farm animals. This disease is endemic in numerous countries in the world, especially in Africa, Asia and South America. In addition, epidemic outbreaks can occur periodically. The presence of this disease in a country may have very severe economic consequences resulting from loss of productivity, loss of weight and milk production in infected herds, and from trade embargoes imposed on these countries. The measures taken against this disease consist of strict application of import restrictions, hygiene controls and quarantine, slaughtering sick animals and vaccination programs using inactivated vaccines, either as a preventive measure at the national or regional level, or periodically when an epidemic outbreak occurs.
FMD is characterized by its short incubation period, its highly contagious nature, the formation of ulcers in the mouth and on the feet and sometimes, the death of young animals. FMD affects a number of animal species, in particular cattle, pigs, sheep and goats. The agent responsible for this disease is a ribonucleic acid (RNA) virus belonging to the Aphthovirus genus of the Picornaviridae family (Cooper et al., Intervirology, 1978, 10, 165-180). At present, at least seven types of foot-and-mouth disease virus (FMDV) are known: the European types (A, O and C), the African types (SAT1, SAT2 and SAT3) and an Asiatic type (Asia 1). Numerous sub-types have also been distinguished (Kleid et al. Science (1981), 214, 1125-1129).
FMDV is a naked icosahedral virus of about 25 nm in diameter, containing a single-stranded RNA molecule consisting of about 8500 nucleotides, with a positive polarity. This RNA molecule comprises a single open reading frame (ORF), encoding a single polyprotein containing, inter alia, the capsid precursor also known as protein P1 or P88. Protein P1 is myristylated at its amino-terminal end. During the maturation process, protein P1 is cleaved by protease 3C into three proteins known as VP0, VP1 and VP3 (or 1AB, 1D and 1C respectively; Belsham G. J., Progress in Biophysics and Molecular Biology, 1993, 60, 241-261). In the virion, protein VP0 is then cleaved into two proteins, VP4 and VP2 (or 1A and 1B respectively). The mechanism for the conversion of proteins VP0 into VP4 and VP2, and for the formation of mature virions is not known. Proteins VP1, VP2 and VP3 have a molecular weight of about 26,000 Da, while protein VP4 is smaller at about 8,000 Da.
The simple combination of the capsid proteins forms the protomer or 5S molecule, which is the elementary constituent of the FMDV capsid. This protomer is then complexed into a pentamer to form the 12S molecule. The virion results from the encapsidation of a genomic RNA molecule by assembly of twelve 12S pentamers, thus constituting the 146S particles. The viral capsid may also be formed without the presence of an RNA molecule inside it (hereinafter “empty capsid”). The empty capsid is also designated as particle 70S. The formation of empty capsids may occur naturally during viral replication or may be produced artificially by chemical treatment.
Some studies have been done on natural empty capsids. In particular, Rowlands et al. (Rowlands et al., J. Gen. Virol., 1975, 26, 227-238) have shown that the virions of A10 foot-and-mouth disease comprise mainly the four proteins VP1, VP2, VP3 and VP4. By comparison, the natural empty capsids (not obtained by recombination but purified from cultures of A10 foot-and-mouth virus) essentially contain the uncleaved protein VP0; identical results with the A-Pando foot-and-mouth virus are described by Rweyemamu (Rweyemamu et al., Archives of Virology, 1979, 59, 69-79). The artificial empty capsids, obtained after dialysis in the presence of Tris-EDTA and after centrifuging, contain no protein VP4. These artificial capsids are slightly immunogenic according to Rowlands et al., and the natural empty capsids are only immunogenic after treatment with formaldehyde to stabilize them, while the antibody response induced by the natural empty capsids in the guinea-pig is nevertheless inconstant, as noted by the author. Moreover, Rowlands et al. and Rweyemamu et al. do not agree on the need to stabilize the natural empty capsids. For Rweyemamu et al., the absence of treatment with formaldehyde is not prejudicial to the level of antigenicity of the natural empty capsids. The immunogenicity is only tested by the induction of neutralizing antibodies in the guinea-pig.
The expression of the gene coding for the precursor P1 of the capsid proteins by means of a recombinant baculovirus in insect cells is compared with the expression of the gene coding for P1 associated with the protease 3C in E. coli (Grubman et al., Vaccine, 1993, 11, 825-829; Lewis et al., J. Virol., 1991, 65, 6572-6580). The co-expression of P1 and 3C in E. coli results in the assembling of empty capsids 70S. The expression product of these two constructions produces neutralizing antibodies in guinea-pigs and pigs. The titers obtained with the P1/baculovirus construction are low. These same expression products induce partial protection in pigs. However, some pigs protected against the disease are not protected against the replication of the challenge virus. However, the E. coli expression system does not myristylate the proteins and the protease 3C is toxic to this cell. Lewis et al. conclude that fundamental questions relating to the make-up of the virus and the structure of the capsid needed to obtain maximum protection in the animal have not been answered. Furthermore, Grubman et al. state that it would be necessary to stabilize the empty capsids before formulating the vaccine; on this point they agree about the problems encountered with the empty capsids obtained by extraction from viral cultures (see above).
Fusion proteins containing some or all of protein P1 have also been obtained by the use of viral vectors, namely a herpes virus or vaccinia virus. CA-A-2,047,585 in particular describes a bovine herpes virus used to produce fusion proteins containing a peptide sequence of the foot-and-mouth virus (amino acids 141 to 158 of P1 bound to amino acids 200 to 213 of P1) fused with the glycoprotein gpIII of this bovine herpes virus. Viral vectors have also been used to express stabilized FMDV empty capsid (U.S. Pat. No. 7,531,182). Recently, plants have been investigated as a source for the production of FMDV antigens (US 2011/0236416).
Many hypotheses, research routes, and proposals have been developed in an attempt to design effective vaccines against FMD. Currently, the only vaccines on the market contain inactivated virus. Concerns about safety of the FMDV vaccine exist, as outbreaks of FMD in Europe have been associated with shortcomings in vaccine manufacture (King, A. M. Q. et al., 1981, Nature 293: 479-480). The inactivated vaccines do not confer long-term immunity, thus requiring booster injections given every year, or more often in the event of epidemic outbreaks. In addition, there are risks linked to incomplete inactivation and/or to the escape of virus during the production of inactivated vaccines (King, A. M. Q., ibid). A goal in the art has been to construct conformationally correct immunogens lacking the infective FMDV genome to make effective and safe vaccines.
It has been reported that maternally derived antibodies (MDA) are able to inhibit calves' (under 2 years of age cattle) response to vaccination against FMD (Graves, 1963, Journal of Immunology 91:251-256; Brun et al., 1977, Developments in Biological Standardisation, 25:117-122).
Considering the susceptibility of animals (including humans, albeit rarely), to FMDV, a method of preventing FMDV infection and protecting animals is essential. Accordingly, there is a need for more effective and stable vaccines against FMDV.